/**
 * covert canvas to image
 * and save the image file
 */
module.exports = function() {

  // check if support sth.
  var $support = function() {
    var canvas = document.createElement('canvas'),
      ctx = canvas.getContext('2d');

    return {
      canvas: !!ctx,
      imageData: !!ctx.getImageData,
      dataURL: !!canvas.toDataURL,
      btoa: !!window.btoa
    };
  }();

  var downloadMime = 'image/octet-stream';

  function scaleCanvas(canvas, width, height) {
    var w = canvas.width,
      h = canvas.height;
    if (width == undefined) {
      width = w;
    }
    if (height == undefined) {
      height = h;
    }

    var retCanvas = document.createElement('canvas');
    var retCtx = retCanvas.getContext('2d');
    retCanvas.width = width;
    retCanvas.height = height;
    retCtx.drawImage(canvas, 0, 0, w, h, 0, 0, width, height);
    return retCanvas;
  }

  function getDataURL(canvas, type, width, height) {
    canvas = scaleCanvas(canvas, width, height);
    return canvas.toDataURL(type);
  }

  function saveFile(strData, filename) {
    var save_link = document.createElement('a');
    save_link.href = strData;
    save_link.download = filename;
    var event = new MouseEvent('click', { "bubbles": false, "cancelable": false });
    save_link.dispatchEvent(event);

  }

  function genImage(strData) {
    var img = document.createElement('img');
    img.src = strData;
    return img;
  }

  function fixType(type) {
    type = type.toLowerCase().replace(/jpg/i, 'jpeg');
    var r = type.match(/png|jpeg|bmp|gif/)[0];
    return 'image/' + r;
  }

  function encodeData(data) {
    if (!window.btoa) { throw 'btoa undefined' }
    var str = '';
    if (typeof data == 'string') {
      str = data;
    } else {
      for (var i = 0; i < data.length; i++) {
        str += String.fromCharCode(data[i]);
      }
    }

    return btoa(str);
  }

  function getImageData(canvas) {
    var w = canvas.width,
      h = canvas.height;
    return canvas.getContext('2d').getImageData(0, 0, w, h);
  }

  function makeURI(strData, type) {
    return 'data:' + type + ';base64,' + strData;
  }


  /**
   * create bitmap image
   * 按照规则生成图片响应头和响应体
   */
  var genBitmapImage = function(oData) {

    //
    // BITMAPFILEHEADER: http://msdn.microsoft.com/en-us/library/windows/desktop/dd183374(v=vs.85).aspx
    // BITMAPINFOHEADER: http://msdn.microsoft.com/en-us/library/dd183376.aspx
    //

    var biWidth = oData.width;
    var biHeight = oData.height;
    var biSizeImage = biWidth * biHeight * 3;
    var bfSize = biSizeImage + 54; // total header size = 54 bytes

    //
    //  typedef struct tagBITMAPFILEHEADER {
    //    WORD bfType;
    //    DWORD bfSize;
    //    WORD bfReserved1;
    //    WORD bfReserved2;
    //    DWORD bfOffBits;
    //  } BITMAPFILEHEADER;
    //
    var BITMAPFILEHEADER = [
      // WORD bfType -- The file type signature; must be "BM"
      0x42, 0x4D,
      // DWORD bfSize -- The size, in bytes, of the bitmap file
      bfSize & 0xff, bfSize >> 8 & 0xff, bfSize >> 16 & 0xff, bfSize >> 24 & 0xff,
      // WORD bfReserved1 -- Reserved; must be zero
      0, 0,
      // WORD bfReserved2 -- Reserved; must be zero
      0, 0,
      // DWORD bfOffBits -- The offset, in bytes, from the beginning of the BITMAPFILEHEADER structure to the bitmap bits.
      54, 0, 0, 0
    ];

    //
    //  typedef struct tagBITMAPINFOHEADER {
    //    DWORD biSize;
    //    LONG  biWidth;
    //    LONG  biHeight;
    //    WORD  biPlanes;
    //    WORD  biBitCount;
    //    DWORD biCompression;
    //    DWORD biSizeImage;
    //    LONG  biXPelsPerMeter;
    //    LONG  biYPelsPerMeter;
    //    DWORD biClrUsed;
    //    DWORD biClrImportant;
    //  } BITMAPINFOHEADER, *PBITMAPINFOHEADER;
    //
    var BITMAPINFOHEADER = [
      // DWORD biSize -- The number of bytes required by the structure
      40, 0, 0, 0,
      // LONG biWidth -- The width of the bitmap, in pixels
      biWidth & 0xff, biWidth >> 8 & 0xff, biWidth >> 16 & 0xff, biWidth >> 24 & 0xff,
      // LONG biHeight -- The height of the bitmap, in pixels
      biHeight & 0xff, biHeight >> 8 & 0xff, biHeight >> 16 & 0xff, biHeight >> 24 & 0xff,
      // WORD biPlanes -- The number of planes for the target device. This value must be set to 1
      1, 0,
      // WORD biBitCount -- The number of bits-per-pixel, 24 bits-per-pixel -- the bitmap
      // has a maximum of 2^24 colors (16777216, Truecolor)
      24, 0,
      // DWORD biCompression -- The type of compression, BI_RGB (code 0) -- uncompressed
      0, 0, 0, 0,
      // DWORD biSizeImage -- The size, in bytes, of the image. This may be set to zero for BI_RGB bitmaps
      biSizeImage & 0xff, biSizeImage >> 8 & 0xff, biSizeImage >> 16 & 0xff, biSizeImage >> 24 & 0xff,
      // LONG biXPelsPerMeter, unused
      0, 0, 0, 0,
      // LONG biYPelsPerMeter, unused
      0, 0, 0, 0,
      // DWORD biClrUsed, the number of color indexes of palette, unused
      0, 0, 0, 0,
      // DWORD biClrImportant, unused
      0, 0, 0, 0
    ];

    var iPadding = (4 - ((biWidth * 3) % 4)) % 4;

    var aImgData = oData.data;

    var strPixelData = '';
    var biWidth4 = biWidth << 2;
    var y = biHeight;
    var fromCharCode = String.fromCharCode;

    do {
      var iOffsetY = biWidth4 * (y - 1);
      var strPixelRow = '';
      for (var x = 0; x < biWidth; x++) {
        var iOffsetX = x << 2;
        strPixelRow += fromCharCode(aImgData[iOffsetY + iOffsetX + 2]) +
          fromCharCode(aImgData[iOffsetY + iOffsetX + 1]) +
          fromCharCode(aImgData[iOffsetY + iOffsetX]);
      }

      for (var c = 0; c < iPadding; c++) {
        strPixelRow += String.fromCharCode(0);
      }

      strPixelData += strPixelRow;
    } while (--y);

    var strEncoded = encodeData(BITMAPFILEHEADER.concat(BITMAPINFOHEADER)) + encodeData(strPixelData);

    return strEncoded;
  };


  /**
   * [saveAsImage]
   * @param  {[obj]} canvas   [canvasElement]
   * @param  {[Number]} width    [optional] png width
   * @param  {[Number]} height   [optional] png height
   * @param  {[String]} type     [image type]
   * @param  {[String]} filename [image filename]
   * @return {[type]}          [description]
   */
  var saveAsImage = function(canvas, width, height, type, filename) {
    if ($support.canvas && $support.dataURL) {
      if (typeof canvas == "string") { canvas = document.getElementById(canvas); }
      if (type == undefined) { type = 'png'; }
      filename = filename == undefined || filename.length === 0 ? Date.now() + '.' + type : filename + '.' + type
      type = fixType(type);

      if (/bmp/.test(type)) {
        var data = getImageData(scaleCanvas(canvas, width, height));
        var strData = genBitmapImage(data);

        saveFile(makeURI(strData, downloadMimedownloadMime), filename);
      } else {
        var strData = getDataURL(canvas, type, width, height);
        saveFile(strData.replace(type, downloadMime), filename);
      }
    }
  };

  var convertToImage = function(canvas, width, height, type) {
    if ($support.canvas && $support.dataURL) {
      if (typeof canvas == "string") { canvas = document.getElementById(canvas); }
      if (type == undefined) { type = 'png'; }
      type = fixType(type);

      if (/bmp/.test(type)) {
        var data = getImageData(scaleCanvas(canvas, width, height));
        var strData = genBitmapImage(data);
        return genImage(makeURI(strData, 'image/bmp'));
      } else {
        var strData = getDataURL(canvas, type, width, height);
        return genImage(strData);
      }
    }
  };


  return {
    saveAsImage: saveAsImage,
    saveAsPNG: function(canvas, width, height, fileName) {
      return saveAsImage(canvas, width, height, 'png', fileName);
    },
    saveAsJPEG: function(canvas, width, height, fileName) {
      return saveAsImage(canvas, width, height, 'jpeg', fileName);
    },
    saveAsGIF: function(canvas, width, height, fileName) {
      return saveAsImage(canvas, width, height, 'gif', fileName);
    },
    saveAsBMP: function(canvas, width, height, fileName) {
      return saveAsImage(canvas, width, height, 'bmp', fileName);
    },

    convertToImage: convertToImage,
    convertToPNG: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'png');
    },
    convertToJPEG: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'jpeg');
    },
    convertToGIF: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'gif');
    },
    convertToBMP: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'bmp');
    }
  };

}();